Zero valent iron is an effective and economic reagent for removal of heavy metals and for destruction of chlorinated organic compounds in water because of its high reduction potential. It is known that zero valent iron can be used to recover copper, silver and mercury in water by electrochemical reduction or iron concentration (Case, O. P. 1974. In: Metallic Recovery from Waste Waters Utilizing Cementation. EPA-670/2-74-008, 9–23; Gold, J. P. et al. 1984. WPCF 56:280–286). Other heavy metals such as lead, nickel, cadmium, chromium, arsenic, and selenium can also be removed from water using iron by reduction and precipitation (U.S. Pat. Nos. 4,565,633; 4,405,464). Uranyl (UO2+2) and pertechnetate (TcO4−) can be effectively removed by iron through reductive precipitation (Cantrell, K. J. et al. 1995. J. Hazard. Mater. 42:201–212). Zero valent iron has also been used to remediate nitrate-contaminated water (Zawaideh, L. I. and T. C. Zhang. 1998. Wat. Sci. Tech. 38:107–115). Iron is also known to be effective for dechlorination of toxic organic compounds such as carbon tetrachloride and trichloroethylene (Gilham, R. W. and S. F. O'Hannesin. 1994. Groundwater 32:985–967; Helland, B. R. et al. 1995. J. Hazard. Mater. 41:205–216).
Zero valent iron in powder, granular, and fibrous forms can be used in batch reactors, column filters, and permeable reactive barriers installed in groundwater aquifers for water treatment and metals recovery. However, iron particles in a filter rapidly fuse into a mass due to formation of iron oxides and deposition of the heavy metals. This fusion significantly reduces the hydraulic conductivity of the iron bed. To solve this problem, a mixture of iron and inert material such as sand has been used in filter columns (Shokes, T. E. and G. Moller. 1999. Environ. Sci. Technol. 33:282–287). The mixed bed cannot be backwashed because iron and sand will be separated into different layers. Stirrers, rotating discs, and revolving drum reactors have been tested to keep the iron in motion in order to prevent the fusion of iron particles (Strickland, P. H. and F. Lawson. 1971. Proc. Aust. Inst. Min. Met. 236:71–79; Fisher, W. 1986. Hydrometallurgy 16:55–67). However, the mixing processes reduce the effectiveness of iron filters and increase wear of the reactors. A fluidized bed column has been developed to remove copper from highly acidic wastewater (U.S. Pat. No. 5,133,873). This process requires utilization of high flow rate and very fine iron powder (i.e., 200 to 950 micrometers) in the filter.
Conventional treatment processes for removal of organic compounds and heavy metals from water are generally based on chemical precipitation and coagulation followed by conventional sand filtration (Dupont, A. 1986. Lime Treatment of Liquid Waste Containing Heavy Metals, Radionuclides and Organics, 7th edition, Washington D.C., pp. 306–312; Eary, L. E. and D. Rai. 1988. Environ. Sci. Technol. 22:972–977; Cheng, R. C. et al. 1994. J. AWWA 86:79–90). Sand filtration alone is not effective in removing heavy metals, especially arsenic and chromate, mainly because sand filter media have a low sorptive capacity for heavy metals. However, if the sand surface of the filter is coated with iron or aluminum hydroxide, the adsorption capacity of the filter media can be significantly enhanced (Meng, X. G. 1993. Effect of Component Oxide Interaction on the Adsorption Properties of Mixed Oxides, Ph.D. Thesis, Department of Civil and Environmental Engineering, Syracuse University, Syracuse, N.Y.).
In column studies, research has shown that cationic metals (Cu, Cd, Zn and Pb) can be removed effectively by sand and granular activated carbon coated with ferric oxide (Benjamin, M. 1992. Metal Treatment at Superfund Sites by Adsorptive Filtration, EPA/540/F-92/008; Jarog, D. et al. 1992. Adsorption and Filtration with Oxide-Coated Granular Activated Carbon, ACS Meeting, San Francisco, Calif., pp. 711–714; Edwards, M. and M. Benjamin. 1989. J. Water Pollut. Control Fed. 61:1523–1533). However, during these processes, sand and activated carbon have to be coated periodically prior to their placement in the filter. Further, the adsorptive capacity of the ferric oxide coating is much lower than that of fresh ferric hydroxide precipitate.
Microfiltration (Martin, J. F. et al. 1991. J. Air Waste Manage. Assoc. 41:1653–1657) and adsorption and magnetic filtration (Chen, W. Y. et al. 1991. Res. J. Water Pollut. Control Fed. 63:958–964) have also been studied as means of removing heavy metals from water. The microfiltration process includes precipitation and filtration in two steps. The main difference between this process and the traditional precipitation and filtration treatment is that the heavy metal precipitates are removed directly through a membrane filter, eliminating the coagulation step. In the adsorption and magnetic filtration process, heavy metals are adsorbed onto fine magnetic particles coated with ferrihydrite. The magnetic particles are then collected using a magnetic filter. Finally, the magnetic particles are regenerated by metal desorption and then reused.
Dermatas and Meng (1996. Removal of Arsenic Down to Trace Levels by Adsorptive Filtration, 2nd Specialized Conference on Pretreatment of Industrial Wastewaters, Athens, Greece, pp. 191–198) tested an adsorptive filtration process for selective removal of arsenic from water. The process involved injection of ferric solution into the top layer of the sand bed or within the sand filter. The stipulated mechanism responsible for removal of arsenic is the coating of sand surfaces with ferric precipitate and subsequent adsorption of arsenic. A direct filtration process has been used for the treatment of source water (G. P. Treweek, J. AWWA, February, 96–100 (1979); M. R. Collins, et al, J. Environ. Eng., 113(2), 330–344. (1987); J. R. Bratby, J. AWWA, December, 71–81 (1988)). The direct filtration process included addition of coagulants to the water followed by flocculation and filtration. A flocculation time or hydraulic detention time of longer than 10 minutes was needed which requires the installation of a large flocculation reactor prior to the sand filter.
A water filtration device has now been developed for removal of heavy metals and organic compounds, such as pesticides, from drinking water, waste water and soil washing solutions. The process for filtering water via this device is based on use of a vibrating iron bed filter and a sand filter.